Overexpression of the Her-2/neu protein and amplification of the Her-2/neu gene are are the most widely accepted prognostic indicator of aggressive malignant behavior of invasive breast carcinoma. Accurate detection of Her-2/neu overexpression is critical in identifying patients suitable for serotherapy with humanized monoclonal antibody (Herceptin). Fluorescence in situ hybridization (FISH) has been found to be more accurate than immunohistochemical staining. A chromogenic in situ hybridization assay has been developed to detect amplification of the Her-2/neu gene in paraffin-embedded invasive breast carcinoma sections. Unlike FISH, which requires fluorescent optics not routinely available in many pathology laboratories, the assay is read in the conventional brightfield microscope. Detection is based on autometallographically enhanced Nanogold, a highly sensitive method giving a dense black signal, fully compatible with commonly used stains. Excellent concordance was found with FISH, histologic grading and mRNA:rnRNA hybridization for a series of 101 breast carcinomas. We now propose to add a secondary assay for chromosome 17 polysotomy, rnRNA expression or concomitant oncoprotein detection as an internal control to resolve borderline amplification cases. After design optimization, it will be ported to an automated slide staining instrument. Once performance is optimized, multi-center clinical trials will be conducted to support application for pre-market approval.